This invention relates to alloys which contain cobalt and which are resistant to wear and to surfacing and welding consumables which are capable of depositing such alloys.
Wear-resistant alloys based on the Co-Cr-W-C quaternary system are well known. They comprise a dispersion of hard carbide particles within a strong corrosion-resistant solid solution, rich in cobalt. The strength of the solid solution is such that only moderate volumes of carbide need be employed to achieve a given bulk hardness at both room and elevated temperature. Material toughness is thus appreciable.
The balance between solid solution strength and carbide level and thus between hardness and toughness of these alloys is such that they are able to withstand wear under both moderate and extreme conditions of temperature and stress (static and cyclic).
A range of such alloys is available, varying in room temperature hardness from approximately 300 VPN to 700 VPN, to suit different applications.
Typically, alloys of hardness of approximately 400 VPN contain 1 wt % carbon, 26 wt % chromium and 5 wt % tungsten, whilst those of hardness approximately 600 VPN contain 2.5 wt % carbon, 33 wt % chromium and 13 wt % tungsten. Not only do the chromium and tungsten contribute to the strength of the solid solution, they also act as carbide formers. The high strength of the cobalt-rich solid solution is associated with a high stacking fault density therein.
In recent years, cobalt has become an expensive commodity. The commercial need to reduce the level of cobalt in such alloys, whilst retaining their excellent wear characteristics, is therefore great.
Merely to substitute iron and nickel for some of the cobalt is an unsatisfactory expedient. A reduction in cobalt level in these alloys and a corresponding increase in iron or nickel bring about a reduction in solid solution strength and bulk hardness.
To compensate, increased levels of carbon (and hance carbide) can be employed. Many reduced cobalt alloys contain increased levels of carbide. The toughness of such materials and their ability to withstand severe wear situations are, however, limited. One example of such an alloy is a composition containing 37% by weight of cobalt and nickel; 10% by weight of molybdenum and tungsten; 23% by weight of iron; 27% by weight of chromium and 2.7% by weight of carbon.
There is a large number of published patent specifications that relate to alloys containing cobalt in smaller proportions than in conventional cobalt-based hard metal alloys. Examples of published patent specifications relating to steels containing nickel and cobalt are Austrian Pat. specification No. 136 854, German Pat. specification No. 160 410, and U.K. Pat. specifications Nos. 1,358,284 and 1 381 170. Such alloys, because they are steel, inherently lack the hardness of hard metal alloys based on cobalt. Examples of published patent specifications relating to nickel-based alloys (or alloys containing more nickel than cobalt and iron) which also contain cobalt and iron and U.S. Pat. Nos. 3,692,501 and 3,700,427, U.K. Pat. specifications Nos. 1,245,158, 880,805, 1,336,406 and 803,253, and Japanese specification No. 53-147615. U.K. Pat. specifications Nos. 424,463 and 1,064,109 both describe a range of compositions for hard metal alloys which allows for the cobalt level to fall well below that in conventional cobalt-based hard metal alloys and which also includes nickel and iron. However, the preferred alloys in both specifications contain over 45% by weight of cobalt. The proportions of iron and nickel that are disclosed are suitable more for an alloy containing 45% or more by weight of cobalt than for one containing less than 45% by weight of cobalt. U.K. Pat. specification No. 579,479 relates to a ferrous-based alloy for hard facing purposes containing 10 to 30% cobalt and 13 to 17% nickel. Japanese patent specification No. 53-14411 relates to a heat resistant alloy containing from 20 to 40% by weight of cobalt, from 10 to 30% by weight of nickel, and some iron. However, it contains less than 4% of molybdenum and tungsten.